Access Control & Windows Server 2008: "Some or all identity references could not be translated"

Hi,
I am trying to set ASP.NET folder permissions for a particular folder in a Windows Installer using the Access Control mechanism.
So far, I've managed to get it working on XP & IIS 5.1. But if I'll try to install the application on Windows Server 2008 I'll get the
exception "Some or all identity references could not be translated". Is there something I should change in my code? 
 
 Here is the code:
 
// This works on XP & IIS 5.1
AddDirectorySecurity(DirectoryPath, System.Environment.MachineName + @"\ASPNET", FileSystemRights.FullControl, AccessControlType.Allow); .............................................
.............................................. // This doesn't work on Windows Server 2008 & IIS 7 and causes the exception "Some or all identity references could not be translated"
...